1. Field of the Invention
This invention relates to cavity sealing articles having improved sag resistance.
2. Description of Related Art
Channels are commonly found in the body structure of automobiles, boats, aircraft, other vehicles, structures (both land and marine), and the like. It is desirable to seal the cavities in such channels against the passage of gases such as air or fumes, moisture, fluids, particulates, etc. As the automotive industry is especially concerned about sealing such cavities in automobiles, the present invention will be discussed primarily in this context.
During the fabrication of automobiles, many body components contain cavities that require sealing to prevent the ingress of moisture and contaminants that can cause corrosion of the body parts. This is especially true with unibody structures, where a heavy frame is replaced by a structurally designed space frame that inherently presents a number of moisture- and contaminant-collecting cavities. These cavities also serve as passages through which road and engine noise and other sounds may be transmitted during operation of the vehicle. For example, the A, B, and C pillars of a vehicle represent elongated cavities that can collect moisture and contaminants and can also transmit sounds that can then radiate into the passenger compartment unless the cavities are at least partially filled with a sealant material. There are other irregular cavities in a vehicle body that desirably are sealed to prevent moisture and contaminants from entering that area and being conveyed to other parts of the vehicle body.
A currently favored technique is the use of a heat-activated foaming sealing material. Typically, a mass of a material capable of expansion (foaming) at elevated temperatures, i.e. a thermoplastic mixture containing both a heat-activated foaming agent and a heat-activated crosslinking agent, is placed on a bracket or other mechanical support or holder, usually made from sheet metal or a molded high temperature thermoplastic, that is capable of being mechanically fastened within the cavity. Because most automobile bodies are now coated by total immersion in phosphating, rustproofing, electrocoating, and other paint baths to ensure that the interiors of all open cavities are coated, the sealing article (the bracket, together with the mass of foamable material), should not fill the cavity cross-section before foaming, so that the coatings may enter the cavity during immersion, flow around or through the sealing article, and drain from the cavity afterwards. As the automobile body is passed through an oven to cure the coating to the metal of the body, the foamable mass expands to fill the cavity cross-section and to seal against the walls of the cavity.
A limitation to this technique occurs when the cavity is in a non-horizontal (i.e., vertical or oblique) channel, such as the A, B, or C pillars of automobiles. During the foaming process, the expanding polymeric mass has a tendency to sag, instead of expanding laterally across the width of the cavity to fill the cavity. This problem is illustrated by reference to FIGS. 1a and 1b. FIG. 1a shows a conventional cavity sealing article 40 (unexpanded) comprising a holder 41 and a foaming sealer 42, for sealing a cavity 10 defined by first and second wall members 12 and 14. A barbed insert 27 passing through a hole (not shown) in wall member 12 holds article 40 at a pre-determined location within cavity 10, in this instance with the sealer 42 side down. When article 40 is heated to expand sealer 42 (FIG. 1b), the molten material of sealer 42 sags because of gravity-induced melt flow, resulting in imperfect sealing of cavity 10.
The problem is more common with formulations of foaming material having low expansions ratios (less than 1.5) and moderately low viscosity in the melt. Cavity sealing articles made with low expansion ratio material tend to have a greater width of foaming material unsupported by the holder. While low viscosity materials have the desirable characteristics of better gap filling capability and better wetting of the cavity wall surfaces (leading to better adhesion thereto), their lower viscosity makes them inherently more susceptible to sagging.
Conversely, if one uses a foamable material which has been crosslinked prior to the expansion step, or is formulated to have high melt viscosity, the sagging situation may be abated to a certain extent, but at the expense of decreased melt flow (gap filling) and wetting. Thus, it is difficult to solve this problem employing only a materials formulation approach.
Hypothetically, one can largely avoid the sagging problem by ensuring that article 40 is always installed with the sealer side up. However, even in this orientation the unsupported edges may still sag. Further, in practice it may not be possible to always have the sealer side up, either because of other design constraints or because of the possibility of human error.
Kitoh et al., U.S. Pat. No. 4,898,630 (1990), recognized this problem (see FIGS. 2-4 therein) and proposed a solution in the form of a composite foaming article, comprising a first sealing member which has an expansion ratio of 1-2xc3x97 and a second sealing member which has an expansion ratio of 6xc3x97 or greater, the second sealing member being laminated to the first sealing member.
Takabatake, U.S. Pat. No. 5,642,914 (1997) and Cydzik et al., U.S. Pat. No. 09/013,400, filed Jan. 26, 1998, disclose a construction in which the foamable material is a relatively thin strip of material surrounding the edges of the bracket. Because there is a smaller amount of material, the sagging problem is generally avoided. However, the absolute amount of expansion in a particular direction is given by the product of the linear dimension of the foamable material in that direction and the expansion ratio. Since the former is small, the absolute amount of expansion (i.e., gap filling capability) will be correspondingly small.
Thus, it is desirable to develop a cavity sealing article immune to sagging problems, which also has a large gap filling capability.
Accordingly, we provide an article for sealing a cavity, comprising:
(a) a planar holder having first and second faces, for holding the article at a predetermined position within a cavity to be sealed, and
(b) a sealer comprising a foamable polymer composition; the sealer having a planar face substantially coextensive with the first face of the holder and in face-to-face contact therewith and further having a peripheral portion which circumscribes the holder in the plane thereof and covers the perimeter of the second face of the holder.